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Is the turning motion of a banked airplane caused by true centripetal force?
Virtually every explanation I have read of the aerodynamics of a turning airplane ascribe the turning motion simply to centripetal force, without further comment.  I'm hoping someone can check my understandings and conclusions whether this is true or not.
As I understand it, centripetal force is defined as the net of the forces on an object that keep it on a circular path.  Put another way, it's the resultant of the force vectors on the object.  Further, centripetal force always acts in a direction orthogonal to the object's direction of motion.
Now, with a banked airplane, the horizontal component of lift, in and of itself, will simply create a lateral force that introduces sideslip. The heading of the airplane must be continuously changed in order to change the direction of that lateral force, and thereby change the force vector from a lateral one to a radial (centripetal) one. This is done independently with the addition of yaw, either coordinated via rudder input, or uncoordinated via weathervaning.
Assuming all this is correct (again please check me), is it mathematically correct to state that true centripetal force is created by the resultant of the force vectors here (the lateral forces adding sideslip and the rotational forces adding yaw)?
Thank you very much for your help.
(EDIT: Through the answers below, I understand now that my third paragraph above is incorrect.  The horizontal lift vector remains perpendicular to the direction of movement (relative wind), not the airplane's heading.  Therefore, while yaw is necessary to maintain coordinated flight, the rotational force vectors behind it are not addends in the centripetal force calculation.)

Centripetal force is defined as the component of the total force acting on the object that is causing it to follow a circular path. It is the force exactly perpendicular to the velocity vector.
To create a circular path, the force needs to turn with the velocity. If you have a force that keeps its spatial orientation, less of it will be centripetal and more of it will just accelerate the object as it moves and the resulting path will be parabolic (e.g. the ballistic curve under gravity).
The horizontal component of lift always contributes wholly to the centripetal force, but this is again quirk of the definitions: lift is the component of the aerodynamic force on the wing that is perpendicular to the relative wind and drag is the component that is parallel to it. So analysing the situation in the reference frame of the air mass, relative wind is the velocity vector.
Now if the plane didn't turn with its velocity vector, it would just gain side-slip, the lift would decrease—banking into side-slip reduces the angle of attack—and the aircraft would stop turning—it's flight path does turn initially, even if the body does not—and settle in sideways flight. So we need to make the aircraft rotate with the turn.
Now just like linear motion, rotating motion stays constant¹ unless acted upon by a moment of force (a.k.a torque). So when initiating the turn, a yawing moment of force needs to be created to also initiate the rotation.
A moment of force may be created by a pair of force of opposite direction and different action lines, so it does not have to cause any linear acceleration, but in case of aircraft usually an initially unbalanced force is created on the tail that starts the rotation, but also accelerates the aircraft sideways until balanced by drag on the fuselage. Note that this force is out of the turn (and up), so it reduces the centripetal force a bit.
But in a steady turn the torque is zero. Or oscillating around zero as the correct angular velocity is maintained via feedback loop. So the horizontal component of lift is the only centripetal force in a coordinated turn. In not coordinated turn, some side force on the fuselage is added or subtracted depending on whether it is a slip or skid.

¹ Angular momentum is conserved but the angular velocity does not have to be. Depending on the axis of rotation and mass distribution (moment of inertia) you may get strange phenomena like the Dzhanibekov effect.

"lift is the component of the aerodynamic force on the wing that is perpendicular to the relative wind" This point is key to clearing up my confusion. In fact, the airplane's orientation around its vertical axis is somewhat irrelevant here.  Since the horizontal lift is always pulling the airplane perpendicular to the relative wind, and therefore (essentially) perpendicular to its direction of movement -- there is a continuous deviation in the direction of movement -- a curved path.  Is that correct?

I interpret centripetal force as a force that is external to the body acting on it. Either a tension force; a string, a magnetic force, or gravity, or a compression force, the walls of a circular chamber with a motorcycle driving around it. Is it still centripetal force if the forces that create the arc of movement about a center axis originate within the body itself? A control line model a/c goes in a circle because of the lines, and airplane goes in a circle because of moments and velocities generated by its own redirected forces - the radius of arc replicates the model, but is coincidental.

@DaveWalzer, yes, it is correct. The airplane's orientation is irrelevant except for the fact that it determines the angle of attack and therefore the amount of lift (and drag) produced.

@JohnK, motion of a (rigid) body can only be affected by forces acting on it, which means they are either caused by some other body or are inertial (but the only inertial force that acts towards some centre is gravity and that is incidentally also caused by another body). For aircraft, the aerodynamic forces are air acting on the airplane, so they don't “originate within the body itself”.

@JanHudec but the orientation of the forces making the airplane fly in a circle originates from the orientation of the body to make aerodynamic forces act upon it (banking and yawing), not from its relationship to a force acting in relation to an axis point.  This is why I have a problem with the definition of the forces involved in a turn as centripetal.  I can chain a car to a bolt in the ground and watch it drive in a circle forced by the chain. Or I can get in the car and make the same circle by steering it; does that mean the lateral force made by the front wheels is a centripedal force?

@JohnK, the orientation and magnitude of the forces depends on the orientation of the body, but the component towards the centre of the turn is still called a centripetal force. And because of how lift is defined, it is the horizontal component of lift (plus/minus some small contributions from the other parts of the aircraft).

@JanHudec ok when I look up the definition in the Oxford dictionary it just says any force that results in a circular motion, so now I get your point  Other sources gave me the impression it had to come from an "externally" applied force.

@John K "force within the body itself vs force acting on the body" is an interesting concept to ponder.  The "control line circle" led me to think it is actually the tensile resistance to stretching that is the "centripetal force".  Notice with a spring scale we can actually measure it in pounds the same way!  A car turning with its tires would have tire friction with the road as the "centripetal force".  A skidding aircraft could actually use a thrust component to turn as well.  It seems a planes thrust is merely acting on its own body, but where the force comes from doesn't matter.

If the airplane banks without a change in pitch, then it will accelerate sideways and build up a sideslip angle while at the same time losing altitude because the cosine of the lift vector will be too small to counterbalance all weight. This downwards motion will increase the angle of attack a bit so the airplane settles at a higher angle of attack, a slight descent speed and a growing sideways motion.
In order to start a turn, the elevator position has to change, too, so the tail lift is reduced (or its downforce increased) and the airplane starts a pitching motion. Also, the rudder should be a bit off-center to start and maintain a yawing motion, however, with sufficient lateral stability this yawing also happens with a bit of sideslip angle at the tail.
It is maybe easiest to view a turn at the extreme bank angle of 90°, forgetting the need to keep some of the lift vector in the vertical direction. Now the circle is actually an inside loop in the horizontal plane. Every turn is a combination of a horizontal loop and a yawing motion.
Now the difference between banked flight and a turn should become clearer: The lift vector has to act at a more rearward location for a turn to happen! It's backward shift must compensate for the pitch damping resulting from the pitching motion which is part of every turn. If that compensation does not happen, pitch damping will stop the rotation and instead result in a sideways acceleration.
With the proper backward shift, the horizontal component of lift is indeed the centripetal force wich causes the airplane to turn.

"If the airplane banks without a change in pitch, then it will accelerate sideways and build up a sideslip angle" -- this essentially asserts that the quality of our pitch control inputs as we enter a turn (i.e. do we hold airspeed (or should it be altitude?) constant?) substantially affects the behavior of the slip-skid ball.  Langewiesche alleges such a thing, a likewise Pagen in his hang gliding tutorials, but in extensive experiments in all sorts of aircraft, I've never observed this alleged correlation to be true.  Could be grounds for another ASE question.

It needs to build up a little bit of sideslip for dihedral effect to work.  Fin sizing has to find a compromise between generating a yaw response to the sideslip too early and creating a spiral tendency, and too late and causing an airplane that slithers about the sky.

A turn in level flight can be modeled as an earth referenced circle.  Therefor any forces that move the plane in curved flight around that circle can be broken into forward and inward vectors.  The inward vector is centripetal force.
It is important to stay with the earth referenced result of forces created by the aircraft, because there are many ways (some better than others) to turn an aircraft in a level circle.
The best way is the "coordinated" turn, where lateral (bank/slip with ailerons) and rotational (yaw/skid with rudder) are balanced, keeping the G forces (ball) towards floor of aircraft.
However, flying the same circle with worst technique in history will produce the same (earth referenced) centripetal force.
So, that leaves one with mass, radius, and angular velocity to determine forces mathematically.  Poor technique simply creates more drag, using more fuel.
Two formulas for centripetal acceleration:
Ac = v$^2$/ r and Ac = r x (angular velocity)$^2$

There are some subtle issues at play here.
I'm not sure that your question is really recognizing the difference between an established turn and a developing turn, i.e. a turn entry.
The fundamental characteristic of a turn is a curvature in the flight path, and the fundamental cause of a curvature in the flight path is a net centripetal force.  The primary effect of banking the wing is to generate a centripetal force that curves the flight path, just as the pull of the sun's gravity causes a planet to follow a curving path through space.  It's misleading to think of the wing's lift vector as primarily acting to drive a sideslip, as one of your statements seems to suggest.
Yet you are right that turning flight often involves some sideslip, or some forces generated to eliminate sideslip, and this does have some effect on the net balance of forces acting on an aircraft in turning flight.
I'd suggest that a good way to approach this question is by looking at three different "models" or paradigms, each more refined than the last.
Model 1 -- in the absence of pilot rudder inputs to the contrary, an aircraft naturally tends to stay "coordinated", i.e. naturally tends to point directly into the relative wind, so that there is zero sideslip.
Model 2 -- when entering a turn, yaw rotational inertia tends to hold the nose on its original heading even as the flight path starts to curve, which causes some sideslip.  Adverse yaw associated with rolling also contributes to these effects.  Appropriate rudder inputs can cancel these effects and keep the nose of the aircraft pointing directly into the relative wind, so that there is no sideways airflow over the fuselage and vertical fin.  These effects are absent once the turn is established, so the rudder input need not be continued after this point.  Note that the net yaw torque must be zero in a steady-state established turn-- in an established turn, the yaw rotation rate is constant, so there is no longer any need to apply a net yaw torque to overcome the aircraft's yaw rotational inertia.
Model 3-- even in an established turn, subtle aerodynamic effects often tend to cause some slip or skid, unless the pilot holds a corrective rudder input.  Even though the net yaw torque must be zero in an established turn, the deflected rudder or the sideways airflow over the fuselage and vertical fin may still generate a horizontal force component, that adds or subtracts to the horizontal force generated by the banked wing.
We can see from model 3 that even in an established turn, it will often be the case that either the rudder is slightly deflected, or the vertical fin is meeting the airflow at a slight angle.  This does generate some sideways force, which does add or subtract to the net centripetal force generated by the aircraft.  Therefore the net centripetal force generated by the aircraft is not exactly equal to the wing's lift vector times cosine (bank angle).  This may have been what you were getting at with your statement "the true centripetal force is created by the resultant of the force vectors here (the lateral forces adding sideslip and the rotational forces adding yaw)".
But there's no conflict between the idea that the net centripetal force generated by the aircraft is not exactly equal to the horizontal force component generated by the banked wing, and the idea that a turn is driven purely by the net centripetal force generated by the aircraft.  Despite the complexities we've been exploring here, the answer to the question posed by your actual title, "Is the turning motion of a banked airplane caused by true centripetal force?", is a definitive yes.
In practice, in a coordinated turn, the force from the banked wing does dominate the overall force balance, dwarfing the force from the deflected rudder or the vertical fin.  Thus the total centripetal force is almost exactly equal to the wing's lift vector times cosine (bank angle).  That's why for most practical purposes, we assume that when the slip-skid ball is centered, the aircraft is pointing directly into the relative wind, so that the fuselage is generating no aerodynamic sideforce.
One instance where this is strikingly not true, is when we are dealing with a conventional twin-engine prop plane with one failed engine.  Here the rudder must be deflected so strongly to cancel the yaw torque from the one good engine, that it makes a significant contribution to the net horizontal force acting perpendicular to the flight path.  This can drive a turn toward the failed engine.  The most efficient way to counteract this force and bring the net horizontal force to zero is to bank the aircraft (typically about 5 degrees) in the opposite direction.  When the optimal bank angle is selected, the aircraft will fly in a straight line, even with no sideslip as measured by a yaw string at the nose.  If any other bank angle is selected (or if the wings are level), if the aircraft is flying in a straight line, then sideslip (as measured by a yaw string at the nose) cannot be zero, so drag is not being minimized.  In this situation it may be tempting to view the banked wing as trying to drive sideslip in one direction and the rudder as trying to drive sideslip in the other direction, resulting in zero sideslip when the two effects are balanced, but I'd argue that that's not really what is going on.  Rather, the bank is trying to drive a turn in one direction, and the sideforce from the deflected rudder is trying to drive a turn in the other direction-- even though the rudder is directed in the same direction as the aircraft is banked.  When you've wrapped your mind around this, you are starting to understand some the more obscure issues involved in the dynamics of turning flight.
When a sailplane is circling at low airspeed, the rudder often must be deflected toward the center of the turn to keep the yaw string centered.  To accommodate the horizontal force generated by the deflected rudder in this situation, one author has recommended leaving the slip-skid ball deflected about half a diameter toward the inside or low side of the turn.  The argument is made that this minimizes sideslip over the overall length of the fuselage, just as is the case in the twin-engine aircraft with one failed engine.  For more, search the web to find the article in "Soaring" magazine entitled "Circling the Holighaus Way", by Richard Johnson.1
In actual practice, unless we are dealing with a failed engine on a twin- or multi-engine aircraft, or are trying to thermal at a low airspeed in a long-spanned aircraft, in coordinated flight the fin or rudder makes a negligible contribution to horizontal forces acting perpendicular to the flight path, meaning that the net centripetal force can be assumed to be equal to the wing's lift vector times cosine (bank), or to weight times (tangent) bank, and the turn can be assumed to be "coordinated" (in the sense that nose of the aircraft is pointing directly into the airflow) whenever the slip-skid ball is centered.
Footnotes--

This article is currently difficult to find on-line.  Here's a related article by Richard Johnson from the magazine "Kiwi"-- "Do you really want to keep the yaw string centered?"

This answer would be improved by adding the following content:  "More content related to the misconception that banking tends to cause sideslip simply by virtue of the fact that the lift vector has a horizontal component -- and the actual reasons that we do tend to see some sideslip while banked and turning --  https://aviation.stackexchange.com/questions/79417/why-does-a-banked-aircraft-create-sideslip/79418#79418 "


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Why are the airport pressure values for "altitude" (inHg) and "sea level" (mb) slightly different on the NOAA web sites?
If one looks, for example, at the NOAA web site for Denver International Airport we see (at the time of posting this question) the Pressure/Altimeter is 29.78 inHg, the Pressure/sea level is 1006.1 mb and the temperature is 54F.  If we assume 1 inHg is equal to 33.8639 mb, then I would have expected 29.78 * 33.8639 = 1008.4 mb for the sea level value.  But the NOAA web page lists 1006.1 mb.  Why is there (always) a slight difference?
-thanks

I'll update the question to current conditions w/temperature @Bianfable.

I understand that American configured aircraft use inHg in the Kollsman window of their altimeters where as foreign configure aircraft use mb in the same.  Still don't understand the slight difference.  Unless my conversion is incorrect.

The unit is usually written as ‘hPa’ as per SI. ‘mb’ has long been defined to mean the same thing, so it does not matter too much.

The difference has nothing to do with unit conversion (which is correctly done in your question). The difference comes from temperature: the aircraft altimeter assumes a standard temperature lapse rate of 1.98°C per 1000 ft altitude when converting a pressure to an altitude reading. The reference pressure for the altimeter (QNH) is defined such that the aircraft altimeter will show the correct airport elevation when it sits on the ground.
The sea level pressure however takes the actual temperature into account. If this deviates from the expected value in the standard atmosphere, you will see a different pressure value here. The higher the elevation, the more pronounced will these differences typically be. That's probably why you noticed these differences at Denver. In your example, the temperature was 54°F, which is about 12°C, but the expected temperature at Denver is
$$ \vartheta = 15^\circ \text{C} - 1.98^\circ \text{C} \times \frac{5434 \, \text{ft}}{1000 \, \text{ft}} \approx 4.2^\circ \text{C} $$
Here is the full explanation from NOAA:

ALTIMETER SETTING: This is the pressure reading most commonly heard in
radio and television broadcasts. It is not the true barometric
pressure at a station. Instead it is the pressure "reduced" to mean
sea level using the temperature profile of the "standard" atmosphere,
which is representative of average conditions over the United States
at 40 degrees north latitude. The altimeter setting is the pressure
value to which an aircraft altimeter scale is set so that it will
indicate the altitude (above mean sea level) of the aircraft on the
ground at the location for which the pressure value was determined.
The altimeter setting is an attempt to remove elevation effects from
pressure readings using "standard" conditions.
MEAN SEA LEVEL PRESSURE: This is the pressure reading most commonly
used by meteorologists to track weather systems at the surface. Like
altimeter setting, it is a "reduced" pressure which uses observed
conditions rather than "standard" conditions to remove the effects of
elevation from pressure readings. This reduction estimates the
pressure that would exist at sea level at a point directly below the
station using a temperature profile based on temperatures that
actually exist at the station. In practice the temperature used in the
reduction is a mean temperature for the preceding twelve hours. Mean
sea level pressure should be used with caution at high elevations as
temperatures can have a very profound effect on the reduced pressures,
sometimes giving rise to fictitious pressure patterns and anomalous
mean sea level pressure values.

(NOAA - Pressure Definitions)

So, if I find NOAA data on an airport at standard temperature (which I think is 59F) then the conversion should work. I see KIAH was at 59F on Jan 5th @ 9:53am. At that time pressure was recorded at 30.21 inHg and 1022.8.  And 30.21 * 33.8639 = 1023.0. That sounds close enough!

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What protections and feedback does the fly-by-wire of the Boeing 777/787 provide?
I know the Boeing 777 and 787 run on Fly-By-Wire. I was wondering:

Do they have the Alpha Floor or Flight Envelope protections Airbus aircraft have?
Do they have force feedback/force feel? The 737, for example, is connected using physical cables that move the flight control surfaces, so you can feel the yoke trying to move and when there is turbulence, it will actually shake a bit. This can't be felt with an Airbus sidestick. Can you feel this in Boeing Fly-By-Wire aircraft?
When banking (without any pitch input), will the aircraft start to descend, or does the system counteract it like in an Airbus? In non Fly-By-Wire aircraft, they will start to descend a bit when banking, but Airbus Fly-By-Wire counteracts it by adjusting stabilizer trim.

A B737 actually has artificial feedback provided by the “feel and centering unit”, because the hydraulic power drive does not otherwise transfer the force back. I believe it is additionally connected with cables for case of hydraulic failure, which provides some additional feedback, but it is far from direct feedback.

@JanHudec Didn't know that, thanks.

1. Do they have the Alpha Floor or Flight Envelope protections Airbus aircraft have?
The Alpha Floor protection on an Airbus overrides the thrust setting commanded with the thrust levers to TOGA. Boeing does not do this. The autothrottles (if armed) will engage to prevent a stall, but the computer never overrides the actual thrust lever position in the cockpit:

With the autothrottle armed, the autothrottle automatically activates if not autopilot
or F/D is active or and autopilot or F/D is in VNAV XXX, ALT, V/S, or
G/S, and:

speed less than FMC calculated value for one second
thrust below reference thrust
airplane altitude above 100 feet RA on approach, or airplane barometric    altitude 400 feet above airport on takeoff

Note: During a descent in VNAV SPD, the autothrottle may activate in HOLD mode and will not support stall protection.

(Boeing 777 FCOMv2 4.20.9 - Automatic Flight - System Description)
Also, unlike an Airbus, this protection can be disabled by simply disarming the autothrottle using the switch on the MCP.
The Boeing 777 and 787 do however have Flight Envelope Protections. These also work a bit differently than in a modern Airbus, where sidestick inputs are simply ignored or limited by the computers. In a Boeing, the flight envelope protection will use artificial forces on the yokes to provide the following:

Flight Envelope Protection
The flight envelope protection system reduces the possibility of
inadvertently exceeding the airplane's flight envelope. The flight
envelope protection system provides crew awareness of envelope margins
through tactile, aural, and visual cues. The protection functions do
not reduce pilot control authority. The protection functions are
described later in this section and include:

stall protection
overspeed protection
roll envelope bank angle protection.

(Boeing 777 FCOMv2 9.20.5 - Flight Controls - System Description)
It says that the pilot control authority is not reduced because the pilots can always overpower the simulated forces on the yoke and command the aircraft to go beyond the protected flight envelope.
Essentially, the philosophy for the control authority differs:

Airbus will not let you do control inputs that might get you killed.
Boeing will stop you from accidentally making control inputs that might get you killed, but you can override them.

See also:

What are the main differences piloting Boeing vs. Airbus aircraft?
How does envelope protection work in Airbus vs. Boeing aircraft?

2. Do they have force feedback/force feel?
Yes, unlike an Airbus sidestick the Boeing 777 and 787 simulate the forces on the yoke and therefore provide force feedback:

The primary flight control system uses conventional control wheel, column, and
pedal inputs from the pilot to electronically command the flight control surfaces.
The system provides conventional control feel and pitch responses to speed and
trim changes.

(Boeing 777 FCOMv2 9.20.1 - Flight Controls - System Description)
You won't see the yoke shake in turbulence like on a 737, but you will see or feel the autopilot inputs as the yoke moves.
See also: How does the Boeing 777's yoke of both the captain and the first officer have synchronized movement?
3. When banking (without any pitch input), will the aircraft start to descend, or does the system counteract it like in an Airbus?
It will counteract like an Airbus, up to 30° bank angle, so you can fly a turn by rotating the control wheel only:

The PFCs also provide compensation for flap and speedbrake configuration
changes, and turns up to 30° of bank. The PFCs automatically control pitch to
maintain a relatively constant flight path. This eliminates the need for the pilot to
make control column inputs to compensate for these factor. For turns up to 30°
of bank, the pilot does not need to add additional column back pressure to
maintain altitude. For turns of more than 30° of bank, the pilot does need to add
column back pressure.

(Boeing 777 FCOMv2 9.20.10 - Flight Controls - System Description)
The PFCs here are the three Primary Flight Computers.

Note that what Airbus is missing is position feedback, not force feedback, because in normal law the force produced by the surface, not its displacement, is what is proportional to the side-stick command.

@JanHudec You are right of course that the sidestick deflection is proportional to the force, but there is no force feedback, which would require some information to be fed back to the sidestick (minor semantic nitpick).

Well, it is a force feedforward, really. The point is that the force on the actual control surface more closely matches the command given with the side-stick, while the position varies with speed (the real reference value of course comes from the ADIRU). So while there is no feedback at all, saying there is no force feedback is misleading in that it suggests the force applied by the pilot does not correspond to the response of the aircraft where in fact it does.

Thanks, very nice answer @Bianfable

@RalphJ I looked both words up in a dictionary (I'm not a native speaker) and you're right, override is what I meant here.

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Do airline pilots use this high-speed, high-drag technique for losing altitude?
This question and another more recent one got me thinking about this.
Years ago on my MEII checkride the DPE showed me a technique for losing altitude on final when the aircraft was high. Typically, I would reduce power, increase drag (flaps, landing gear, etc), hold nose up to lose speed,   and sink like a rock (if necessary slip). However, what he showed me kind of blew my mind. I rarely do it, except when just having fun.
He said, "Do you remember what happens when airspeed doubles?" I said, "Yes, drag quadruples." More precisely the parasite drag quadruples. Drag $= pv^2$, where $p$ is some constant representing parasite drag and $v$ is velocity.
So, we set up high on final, and he dropped the flaps and descended at $v_{FE}$. I was surprised to find that the maneuver used up a lot of the aircraft's energy, and the aircraft was brought down to the glide path and slowed down with plenty of time for a normal landing.
My DPE was a Captain for Southwest back when Southwest was a small airline. My question is whether this is a technique used by airline captains nowadays?
I don't have access to a B737 to try this out, but in a small aircraft like a Cessna 152 you have a $v_{s_{0}}$ of about 35 and $v_{FE}$ of 85.
The idea is you can descend with full flaps at 42.5 knots and power off and it will sink slowly but in a somewhat nose up attitude. Or, you can put the nose down and speed up to 85 knots and quadruple the drag and get on your glide path sooner so that the last bit of the landing is at a proper glide path, attitude, and speed.

I don't have access to a B737 to try this out Wow man you play big leagues :D

hehe - This title reminds me of spammy ads. "Learn this one simple technique for losing altitude! The FAA hates it!"

I recall that an approach from the west to Denver, Colorado when given a straight shot into approach involved losing 7000 feet in 2 minutes after clearing high terrain, with the aircraft "dirty" and nose down quite a bit.

I should add that when this was done in a heavier twin the angle of descent to achieve Vfe is not as steep or dramatic as in a little C152. So, I imagine that in a slippery airliner you don't need a dramatic down attitude to reach Vfe, and its probably more comfortable than s-turns or a slip for passengers. If the descent is within Vfe, it should be a kosher maneuver. Great answers everyone!

I actually got asked a similar question on a checkride two weeks ago.  Right after I was asked to demonstrate an emergency descent through a hole. I heard the talk about the advantages of the substantially increased drag of the Fowler flap, and was quizzed about impact of drag with higher speeds.  Meanwhile I extended full flaps, and slowed the plane.  Midway through the hole, I was "corrected" that the proper way would be to fly closer to Vfe. I disagreed and pointed out that at a slower speed, in a 60 degree bank, our turn radius was smaller, and we would remain VFR within the hole.
But to answer your question, the procedure depends upon what airline, and what country. Two US carrier captains say that if the procedure isn't in their books, then they can't do it, except in an emergency.  My take is that configuration changes take time, have risk, and flying slow or fast have risks.  So for one US carrier, the answer from two of their senior pilots is, no we do not do that.
As a final note, pilots who tow gliders and fly in drop zones are motivated to get down quickly.  Most will dirty the plane, close the cowl flaps, pull back on the power, and roll in lotsa bank. I do the 60 degree turns with enough airspeed to keep me away from an accelerated stall.
Congrats on your checkride.  ps: The two guys I was talking with about this do not fly for Southwest.

The towplane technique gets you down fast but is very stressful for the engine. It just had to do heavy work at low speed (= gets hot) and now is idle at high speed (= gets cold quickly). If you want to keep the engine for its full 2000 hours between overhauls you better descend at a lower speed.

Perhaps I was not clear enough, because the power is not initially brought to zero, it is just pulled back. By closing cowl flaps and keeping the speed low, but high enough to support a 2g turn, the sink rate exceeds 2000 f/min.  Since club planes are not subject to TBO, we get rewarded when they go beyond TBO.  3000 hours or more is not uncommon prior to a real overhaul.  Cylinders will normally last about 1500 hours. It's really another good example of having a SOP which gets periodically reviewed.

Airline pilots certainly do use gear/flaps/spoilers to descend more quickly if needed. However, SOP typically requires an approach to be stable upon reaching a certain altitude on the approach. This means having speed and airplane configuration set for landing, and being lined up with the runway and on the glideslope. Unstable approaches are more likely to result in landing mishaps.
Some situations such as a runway change may require late changes to the approach. It is up to the crew whether they can safely make this change or if a go-around is the better option. S-turns are another way to lose further altitude.
When Air Canada Flight 143 ran out of fuel, the pilot used his glider experience to perform a forward slip to lose altitude. These sorts of maneuvers would be reserved for an emergency on an airliner.

Good answer, I'd recommend bolding the "only in an emergency" in your last sentence for emphasis.

It's hard to get above Vfe in a Cessna with full flaps once you've slowed down. At least in the lightweight 152s and 172s I used to fly. You could go to 45 deg nose down easily. But as one reply said, that's hard on the engine.
I guess I never tried it at full power, but I got used used to making pretty much all "major" descents with full flaps so I could keep partial power to prevent shock cooling the engine.

In a 152 with full flaps you can get to 85 kts by doing a steeper descent (true, you'll see a lot more ground than usual, but its only for a few seconds while you get to your desired altitude or glide path). I'm not suggesting to get above Vfe because that would exceed maximum flap extended speed.

@Devil07, an a Pawnee converted towplane, the cowl flaps closed and nose low at a high idle keeps the CHT from swinging, and regular performance is -2000 FPM.  Ditto in a Cessna 206 jump plane.  Managing engine cooling rates is very easy, even in the winter months.  If you don't have pax, why not roll into a 60 degree turn?  The higher loading will help the sink rate much more than 1.4g at 45 degrees bank.

G protection on a clean jet ( no flaps/ slats) is HIGHER
typically there’s no negative G protection with flaps/slats extended
Also typical tear and wear of the flaps/slats mechanism is higher when operated near their operating limits ( speed)
F/S usage has limitations ( Max alt, VFe)

Keeping the above in mind, the "normal " procedure would be to use speedbrakes to increase the rate of descent rather than lowering the flaps / slats.
This being said, some line pilot elect to lower the F/S instead of using speedbrakes, when the conditions allow ( smooth air, speed and altitude )
They do this mostly:
- for comfort (to prevent the typical buffeting created by the speedbrakes)
- in anticipation of a further speed reduction imposed by ATC
NOTE : Another way to achieve a higher rate of descent, without compromising the G protection would be to lower the gear.
But this is very noisy, and limited ( typical max speed for lowering the landing gear is 250 knts) and altitude ( usually max FL200)
Using gear & F/S to achieve a higher rate of descent is used to get established on the final glide path when intercepting from above.
This must be must carefully planned.
Indeed any low level flight segment in landing configuration is very penalising in terms of fuel burn

It sure will descend faster in a high drag config in a dirty config in a high speed glide.  Think of it this way:  you’re depleting the energy state of the aircraft much faster.  An airplane a energy state is the sum of its kinetic energy ie how fast the groundspeed is and potential energy ie how high it is above the ground.  As you pointed out as speed increases, drag increases as the square of the speed, and energy is a product of the force applied to an airplane over a given distance, this means you are draining four times as much energy from the A/C’s energy state at twice the groundspeed.  If airspeed is maintained ie pitching for a fast glide without additional power, the energy loss is coming from the potential energy reserves.  As potential energy is directly proportional to altitude, you are going to lose 4x as much altitude descending at twice the given airspeed.
I’ve flown DA-42s and DA-62s, both of which allow the pilot to lower the gear as high as Vne.  For emergency descents in that plane, you simply roll into a 30-45° turn and drop the gear, thence pitch for a speed as high as Vne (typically just short of Vno for simulated emergency descents).  You will get down faster in a high drag state than a low drag state for a given airspeed.
As for doing that in a jetliner with the paying passenger load, well it is always an option in emergency, it would generally be considered a faux pas under normal conditions. Remember the idea is to keep the passengers comfortable and not spooked or frightened.  A far superior solution to the problem would be flight management so that you arrive at certain way points at certain airspeeds and in certain configurations which minimize steep descents, rapid deceleration, etc.

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text:
What is the weight saving of windowless fuselage for cargo aircraft?
What is the weight saving of a windowless fuselage for cargo aircraft?
Does anyone have a rough estimation?

Just as a matter of information, be aware of the fact that many freighters were originally passenger aircraft that have been converted. In such cases, the window ports are typically painted over. Whether anything is done other than to paint them over I do not know.

Let's take a Boeing 737 as a generic example, and spitball some numbers.
I'll make the post community wiki so someone can come along and plug in more accurate numbers.
Aluminum has a density of 2.7 g/cm3
(For our purposes let's assume the skin alloy is "close enough to pure aluminum" because I'm too lazy to look up the proper alloy density).
Plexiglass (just picking a random variety) has a density of 1.18 g/cm3.
The aluminum skin on a 737 is about 0.038 inches (or 0.1cm) thick.
We know a window on a 737 is thicker than the skin. I couldn't find the number but let's go with 0.25" (0.6cm).
If we have a hypothetical one-foot square window (about 930 cm2), the aluminum skin occupying that area would weigh about 251 grams. The Plexiglass equivalent would weigh about 658 grams (407 grams heavier, little more than 2.5 times as heavy).
Figure there are about 70 side windows on a 737 (quick count from a photo of a Southwest plane, not including exit door windows), so that's 28,490 grams or about 64 pounds, just for the plexiglass.
In addition to the plexiglass there's also supporting structure that needs to be added around the window to hold it in place, sealants to seal it against the skin, etc. - these add additional weight. I've no idea what these components weigh, but let's assume it weighs twice as much as the plexiglass, that's about 1.5 pounds per window of additional structure, or 105 pounds, bringing the total weight savings to just under 170 pounds.

170 pounds is not a lot of weight (in passenger terms it's not even one person), but it is that much more cargo you can carry. Also when combined with the fact that you no longer have to maintain the windows (fix pressure leaks in the seals, replace cracked plexiglass, etc.) the savings can add up over the life of an airframe.

Some more spitballing here... please feel free to correct my numbers:
On the subject of the cost savings: a 737 can carry about 27000 lbs payload, and burns about 5500 lbs of fuel / hour during cruise, and about 3000 lbs for a 15 minute climb. Assuming an average flight of 4 hours you get about 25000 lbs of fuel burnt, or almost 1 per pound of weight. This means that the fuel cost of 170 pounds of weight is about 170 pounds of fuel, which today costs (at around \$4 / gallon, and 6.5 lbs/gallon) around \$100.  Note that the cost of fuel varies a lot with location, so this is very approximate.
Now multiply this by the typical life of the aircraft - three flights per day, 7 days per week, for 30 years? That would be over \$3M of savings. And all because your parcels can't look out of the window.

Though I have no idea about the real numbers, the thickness of the alu seems OK, but for the plexi, 6mm are pretty much! I would say it's not more than 3mm, except you are speaking of the overall thickness of all windows. But only one of them has to withstand the pressure and mechanical stress, the rest is e.g. for insulation. So I guess, the difference is even smaller.

@sweber There are multiple panes of plexi, I'm going with 6mm as the sum of all the panes. That number might be a little high, but I don't think so - you can find quarter-inch side windows on some unpressurized light singles (usually as an aftermarket upgrade for soundproofing purposes).

Most of the weight savings will be from eliminating all of the extra material that has to be added to reinforce the windows. I think that weighs more than is estimated here.

@fooot I'm sure it is - I deliberately low-balled that number because I don't have a good reference for exactly how much structure is added (I assumed a doubler plate, an internal frame, and some hardware - but there's probably more). Even with these numbers though the weight savings is noticeable: 170lbs is a lot of mail, for example.

You also have to consider the differences between purpose built cargo aircraft(new build to freighter) vs an aircraft that used to be in passenger service but was changed to freighter service after a conversion, and each program is unique in how they shore up flooring and plug up former windows.

As a side note I belive que main reasons to no put windows on cargo planes are that way they are quiker and cheaper to build. Also its a lot safer to not have windows if you are loaded with a cargo of bricks and it get lose with turbulence

@jean, your building project must be in serious trouble if you're air-freighting in bricks. That's going to burn a big hole in the budget!!

@FreeMan it was anedote based in a real fact. A C-130 crashed  and investigation pointed a possible cause was a load of brick get loose in cargo bay.

@jean, Yowzers!! Of course, C-130 means military, and military means throw all common sense out the window. ;)

By the logic in your last paragraph, you should be able to fly an empty 737 using no fuel at all.  That calculation of fuel per pound of cargo is wrong.

@Adam I specifically said these are spitball calculations (US engineering slang for "gross oversimplification of the facts to illustrate a general concept"). If you would like to source accurate numbers for fuel burn per pound of aircraft weight (and for that matter for the window weight using Peter's notes in the other answer) I would welcome an edit to bring the numbers more in line with reality, but the point is "Fuel spent hauling the weight plus added cost in construction make windows on cargo jets a losing proposition" :-)

To spitball you need some basis.  The equation in the last paragraph doesn't have any.  You simply are equating to things (fuel weight and cargo weight) that aren't necessarily related, or related that way.  The prior set of calculations do have some basis.

The final section assumes that all the fuel is used to carry cargo, and there is no fuel used to move the rest of the aircraft.  When, in fact, a large percentage of the fuel is necessary to move the empty aircraft.  You are confusing marginal and total quantities.

The additional mass due to windows can best be estimated by statistical methods. By comparing the structural mass of windowed and window-less fuselages of otherwise identical aircraft and formulating a numerical equation which fits all cases with the least error, you get formulas like
$$n_{Windows}\cdot 4.641\cdot S_{Window}^{0.976}\cdot(1.0 + \Delta p)^{4.945}$$
where $n_{Windows}$ is the number of windows, $S_{Window}$ is their surface area in m² and $\Delta p$ is the pressure differential of the pressurized fuselage in bar. The resulting mass is in Kilos (kg). This formula is from the LTH (Luftfahrttechnisches Handbuch) and based on 16 aircraft. Standard deviation is 20.3%.
Actual values per window are between 3.5 kg in case of the DC-10-10 and more than 7 kg in case of the Boeing 747-200. Sorry, I have no data for the XL-sized windows of the Boeing 787. If we assume 80 windows in total for a Boeing 737-class aircraft, the total mass of the windows is about 300 kg.
Jan Roskam cites a different formula (called the GD method, originally from General Dynamics). Here window mass is
$$109\cdot(n_{pax}\cdot\frac{1+\Delta p}{100})^{0.505}$$
Now the pressure differential is in psi, the result in pounds and for an aircraft with $n_{pax}$ = 150 passengers the value is 437 lbs or 198 kg.
The window mass takes account not only of the transparent material, but also of the local reinforcement of the fuselage skin. Either doublers are riveted around the window hole, or the whole skin is chemically etched to reduce its thickness, and the area around the window is masked to keep the original thickness. As a rule of thumb, you should add around the circumference what you take away by cutting the hole. Thus, a well-designed fuselage should have a net mass increase roughly equivalent to the mass of the windows themselves.
The Roskam formula is older, and window size and mass have increased over the last decades. Therefore, the newer the aircraft, the heavier its windows.

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text:
How can I estimate the required T/O power for propeller aircraft?
How can I estimate the maximum required take-off power for a propeller aircraft with the following available data?

Wing surface S
Max. lift coefficient in T/O configuration cL
Take-off distance available d (regardless of obstacle clearance etc.)
MTOW
Airfield altitude (let us assume MSL)

Is the data available even enough?

You need takeoff speed (from the lift coefficient, wing surface and altitude) and the time in which you need to achieve it (before you run off the runway). That'll give you the needed acceleration ($a = \frac{\Delta v}{s}$)
From that you can know how much force the prop must generate ($F = ma$).
Add some margin to account for friction losses.


text:
Does one have to practice engine failures in flight training?
Are engine failures practiced normally? What exactly are the reasons? Is there a particular type of failure that is required training for all pilots worldwide?

@Peter I read that.

Just a thought - I'd guess (as a non-power pilot) that engine stalls are usually obvious in their symptoms so are practiced to train the correct recovery response. Aerodynamic stalls may be different, as I think the symptoms can be less obvious. (There have been incidents of experienced glider pilots fatalities, who apparently were never even aware that they were in a stall.) So stall recognition is a slightly different skill to recovery.

Are you asking about engine failures in general, or just engine stalls?  Engine stalls in flight are, if not impossible, so rare I've never heard of one.  What was not answered in the duplicate?

OK, please let us know what is missing in the answers!

The other question is primarily addressing aerodynamic stall, not engine failures, as the body of this question seems to be addressing. While the other question does point out the difference between the two, it doesn't really answer why engine failures (or simulated engine failures) are practiced. That said, there's probably a dupe somewhere that does.

@verve If this question is asking about Engine Failures (a totally distinct concept from stalls, which in aviation always refer to the aerodynamic phenomenon) please edit & retag appropriately and it can be reopened.

@verve are you asking about aerodynamic wing stalls or engine failures?

Engine stalls?!

Are simulated engine stalls ever practiced? How common is it?

Right now we're not sure what you mean by an "engine stall". It sounds like you either mean "engine failure" (i.e. the engine stops; see here) or "aerodynamic stall" (i.e. the wings stop generating lift; see here). If you're thinking about something else, can you find a link to a site or video that shows exactly what you mean?

do you mean jet-engine compressor surges?

@verve I'm going to assume you mean "engine failure" and have edited your question accordingly.  Your terminology is not what we use and is confusing what you actually want.  Forget "engine stalls" and instead pick one of engine failure, aerodynamic stall, compressor stall to use.  "engine stalls" may have meaning in a car, but does not have a meaning in airplanes.  Research the bolded terms and figure out which one of them is what you think "engine stall" means, and then start using that phrase instead.

@casey So simulated power off stalls are not practiced?  Isn't a power off stall like an engine stall?   I don't mean engine failure though...

@verve A power-off stall is a landing configuration idle/low engine power aerodynamic wing stall, to simulate a wing stall on approach to land. The engine is perfectly operational.

@verve It may also help you when you see "stall" to replace it with "loss of aerodynamic lift due to exceededing  the critical angle of attack".

From a general aviation standpoint here in the US, yes. Engine out landings and general procedures are practiced during your PPL training. No particular failure is trained for because there are really only 2 situations, the engine is running or the engine is not running. The procedures for different planes vary but generally you set for best glide, pick a touchdown point (airstrip if you can) and head for it. Once in route you can attempt a restart if you can identify the problem and solve it. on some planes you can feather the propellor and such.
That being said that generally applies to single engine planes, in a twin engine plane there are a few more situations depending on the design of the plane. In a twin you may have a "critical engine" as such engine outs are practiced with that in mind and the procedures are adjusted appropriately.
As for the why...
There are lots of things that can take out an engine like a bird strike, a mechanical failure, a fire etc. The fact of the matter is that it does happen  all be it not that often, you would be a fool not to prepare.
Anecdote:
When I started my training I got curious about piston engines cutting out so I began to ask around. Youtube videos aside, I was only able to actually find one guy who knew a guy that once lost an engine. Engine manufacturing has come a long way since the advent of airplanes and engines are simply more reliable than they once were. Here is a great article that touches on some of the twin engine points about reliability. While it does happen and you should never assume "this wont happen to me" engines are much better than they were 50 years ago when some of the requirements were made for the tests and training standards.

I had an engine failure in 1971 in a Piper Cherokee 140. The failure eventually resulted in an AD being issued for that model of Lycoming engines. Then in the early 1980s I had to shut down the right engine of a Cessna 310 due to a runaway prop. Tear down of the propeller hub revealed solidified oil had jammed the pitch mechanism.

Do Cessna 172's have a single engine?

Counting the number of propellers (not propeller blades) will tell you the answer with most but not all aeroplanes.

The most common cause of modern engine failure is fuel-starvation:  The Pilot ran out of fuel, due to his own mistakes.  Mechanical failure is pretty rare.

Possible reasons of failure include (in no particular order):

Running out of fuel
Not having the fuel turned on - the engine might fail just after you have lifted off - so don't change tanks after your power checks
Fuel pump failure (many low wing aircraft have two pumps)
Mechanical failure of the engine
A propeller blade flying off - you want to stop the engine immediately so that the engine doesn't shake itself out of the aeroplane - if this happened, the aircraft would not be controllable and the end result very bad
In Flight Engine Fire. Not only would you want to stop the engine, you might like to do an emergency descent, which gets the aircraft on the ground as quickly as possible without excess airspeed
Ignition system failure that causes either a full power loss or partial (back firing, rough running)
A passenger touching the throttle or mixture control
Failure of the exhaust system - leaking exhaust is hot and in the engine compartment could cause a fire or failure.
Broken throttle cable leaving the engine at the last power setting (could be too high or too low for a safe landing and/or continued flight).

I am sure there are many more. There might be also other reasons such as lack of daylight, bad weather, or a problem with the aircraft where you may prefer to carry out a forced landing in a field.
What you do about a sudden failure depends how close to the ground you are. You must fly the aircraft first - Aviate - Navigate - Communicate. So if the engine fails at 100 feet you must look where you are going and aim to reach the ground under control, even if that means hitting something. Much better than to lose control, stall or spin and hit the ground out of control - this will likely kill you.
If its an engine failure just after take off. Have your own limits as to when it is safe to turn back. Remember you might be turning back into oncoming traffic so its not always the best option even if you do have enough height. My personal limits are that if I have already turned 90 degrees in the circuit and my height is 500 feet AGL or above then I might consider a turn back. This is something you can practice at altitude with an instructor to get it clear in your mind.
If you have enough height, the first thing to see is if the propeller has stopped turning. Airflow will normally keep it turning so if it has stopped the engine might have a mechanical problem that means it will never re-start. If the propeller is still turning, fly the aircraft first and foremost but it is worthwhile memorising the emergency checklist. This might include (from my memory, i.e. not to be relied upon):
Carb Heat - On
Mixture - Rich (you may have leaned it out too much and caused a failure, or descended with lean mixture making it even leaner)
Fuel Pump - on (maybe the engine driven pump has failed)
Change Tanks in case one is empty or blocked
Throttle position - check it hasn't vibrated to an idle setting (your hand should be on the throttle during take off to prevent this)
Magnetos - maybe one has failed the engine may run well on one or the other.
Don't forget your pre landing checks if it still doesn't start!
When you get closer to the ground close the throttle and forget about it - your attention must be flying the aircraft and landing as safely as possible.
Flying a twin is harder, because you must learn how to fly it on one engine safely (it will pull one way or another) and this takes experience and currency. Otherwise, rather than you flying the aircraft, the live engine will take you to the scene of the accident. Many twins have single engine safety airspeeds (blue line) which if you go below, you cannot have the live engine on full power otherwise you will lose control due to lack of airflow over the control surfaces.
Some pilots don't touch the engine controls after takeoff until at 500 feet, because there is a slightly increased chance of failure if you do change the power setting for example, or pull the mixture instead of the throttle.
A difficult situation can be one where the engine is producing partial power - not quite enough to fly with. You might be tempted to try so you can reach an airport rather than land in a field.
Despite writing all this I feel sure I have missed something, hope this helps anyway.

Oh yes - if you have time, make a Mayday call. This tells air traffic you have a problem and will alert the flying club and emergency services. It may also be useful if you are turning back to the runway to tell others you are landing downwind, dead stick.

Although I mentioned it later in my reply, the one thing I missed in my 10 causes which is common is carb ice. You only have to read accident reports, people die on a fairly regular basis due to engine failures due to carb ice, because if you don't do the regular checks and get carb ice and the engine stops, by the time its melted you may already be dead and be no longer in a position to restart the engine.

On reading this back, I think I've missed one of the big reasons why you would want to train for engine failures .... to practice engine failures with an instructor just after take off. You need to work out rules in your own head as to when its safe to turn back to the runway you have just departed. I would bet that turning back to what feels "safe" but too low to do it kills more pilots than engine failures themselves. Without practice your instincts will tell you to turn back which is usually the wrong thing, many pilots not even making the turn stall spinning instead.

The question of engine failure in Single Engine, Fixed Wing, Piston Engine, Factory Built (SE FW PE FB) aircraft is one where myth overtakes reality.
A quick look at the NTSB.gov website (publications, annual reviews) shows that engine failure from mechanical issues ranks from first to third as the cause of GA accidents in every year since the summary stats were first posted.
Having experienced three engine failures in about 4,000 of flying SE FW PE FB I began to wonder about just how reliable my engine really was.  My engine failures were in three separate engines, three different aircraft, and included one catastrophic failure and two partial power loss events.
All were mechanical failures traceable to maintenance.  None show up in the NTSB data base because all had a happy ending (no injury, no damage, no requirement to report - and therefore invisible to the 'official' tally of engine failures).
In 2014, after my catastrophic engine failure I put together a survey which was posted on several forums and sent to several flight schools.  Six questions asked about total pilot time, logged hours in SE FW PE FB aircraft, loss of power events (both partial and complete loss), and a brief optional narrative of any loss of power event.
At this time I have about 760 responses. FWIW, a very impressive number of pilots have experienced one or more engine failure.  Overall, considering both those who have not (yet) experienced loss of power from any reason, about 10% of all participating pilots have experienced loss of power that required early termination of flight, but only 2-4% of all pilots, more or less, reported an accident where the NTSB required notification.
Of course, the probability one will have an engine failure is related to exposure.  Higher time pilots tend to have more loss of power events.  These data suggest to me that the total number of engine failures in SE FW PE FB aircraft are substantially higher than the NTSB data would suggest... In other words, data shows there is a myth of piston engine reliability.
Practice for engine failure (from any cause!).  You might need those skills.

Your answer may be a little more pleasing to read if it wasn't just a wall of text. Add some paragraphs/formatting and it may add something here.


text:
What is the significance of 104% for throttle power and rotor speed?
I noticed this initially when learning to fly Robinson Helicopters but fail to remember the reason my instructor gave me; this again came up when watching a video about the ISS.

In helicopters (Robinson R22/44), the main rotor typically operates at 104% RPM.
On the space shuttle, after max q, they throttle back up to 104%.

What is the significance of 104%? If this is the maximum, why is this not called 100%, and everything scaled back to fit? This seems the most fit place to ask this, as I have only seen this in aviation / aerospace.

Related on space.SE: Why are rocket engines operating above 100% often considered nominal?

Maybe because 104% is the 11 of aviation?!

Also on [space.se], Which STS mission raised the normal engine throttle above 100%, and what change to the SSME made that possible? (Full disclosure: My own question) (STS is basically the now retired US space shuttle.)

For what it's worth, I've also seen "104%" several times in retro computing demos.

This happens when the maximum output was changed after design.
100% is simply a reference to a certain value. In the space shuttle's case, the engine output was increased after the initial design. Instead of updating the engine value everywhere, they decided to simply keep the initial value constant.
The same goes for N1 in turbo engines. 100% N1 is simply referencing a certain datum defined by the manufacturer. Going over 100% is not necessarily unsafe.

You're on ten here, all the way up, all the way up. You're on ten-where can you go from there? Where? I don't know - nowhere. Exactly!
If we need that extra push over the cliff, you know what we do? Put it up to 11.  Exactly. One louder. Why don't you make ten louder,
make ten the top number, and make that a little louder? These go to 11.

For gas turbines, there is an absolute maximum rotor speed, beyond which the stress levels are unsafe. But the power from the engine depends on the atmospheric pressure and temperature as well as the RPM. For a low and cold takeoff, the safe maximum N1 speed may be less than 100% N1. "100% N1" is typically the design speed at the base point for the International Standard Atmosphere tables and formulas, i.e. sea level altitude and 15C temperature.

These go to 11 (clip referenced in @ivanivan's comment).

I was counting down the minutes to a Spinal Tap reference lol.

Obligatory xkcd reference.

@reirab: but remember - it's impossible to build an amp that goes to 13. The universe simply won't allow it!

@kevin has given the general answer, so I will only give the answer to the very specific question you asked:

What is the significance of 104%

None.
As was explained by Kevin, 100% is the design reference point. Engineers are generally very good at predicting performance, so they will typically only be off by a tiny amount. They are also typically conservative, so they will be off in the lower direction.
This means that the actual performance will typically be just a tiny bit above 100%, and there are not that many numbers that are a tiny bit above 100%. So, it is not surprising that out of the vast number of rockets, airplanes, motors, turbines, engines, etc. you have found two which happen to have the same rated performance figure of 104%.

There is often actually very significant difference between rated performance (what the 100% is based on) and possible performance, though, due to significant safety margins in the designs. For example, the GE9X is only supposed to be rated for 105,000 lbf thrust, but produced 134,300 lbf in testing, breaking the previous world record of 127,900 lbf set by its predecessor, the GE90-115b (which was itself 'only' rated for 115,200 lbf.)

authors: Bob Jarvis - Слава Україні


text:
Can GPS be used in lieu of DME for instrument approaches?
Instrument approach charts list the equipment required to fly them in their name. For this example let's say we have an ILS or LOC/DME 36R approach. This approach requires an ILS receiver for the ILS portion, or both a LOC and DME receiver for the localizer only portion.
Let's also say we're flying an airplane that has an inoperative glideslope receiver and is otherwise /G equipped with an IFR-certified and up-to-date GPS. Can the GPS be used in lieu of DME in this case?

I was going to post this as an answer, but I'm not sure it covers all the points: AOPA article for substituting GPS for DME/NDB

Yes, the details are in FAA AC 90-108:

7. Usage of Suitable RNAV Systems. Subject to the operating requirements in this AC,operators may use a suitable RNAV system in
  the following ways.
(1) Determine aircraft position relative to or distance from a VOR (see first note in subparagraph 7b), TACAN, NDB, compass locator
  (see second note in subparagraph 7b), DME fix; or a named fix defined
  by a VOR radial, TACAN course, NDB bearing, or compass locator bearing
  intersecting a VOR or Localizer (LOC) course

And all fixes, procedures etc. must be loaded from the GPS database, you can't enter any fixes yourself based on lat/long or whatever (section 9(c)(1)).

The short answer is, "Yes, GPS can substitute for DME".
The longer answer, splitting some very fine hairs, is that they are not exactly the same thing, and you should at least be vaguely aware of the differences.
DME literally measures distance, by measuring how long a signal takes to make the round-trip.
GPS only knows the position of the receiver and the position of the fix in the database, and just calculates the distance between.  That means that GPS is using trigonometry and math to calculate the distance between two points on an idealized sphere, while DME is also accounting for slant-distance.
IF the location of the target fix is affected by slant-distance or by some peculiarity of terrain, there may be a tiny discrepancy between DME's measured actual distance and GPS's calculated distance.   But in practice this difference is always going to be extremely small.

That's all correct, but since this is a question about instrument approaches   it's important to consider what the regulations allow, not just the technology

Correct except maybe "on an idealized sphere", GPS and RNAV systems use WGS84 datum, hence an ellipsoid.


text:
When should a pilot use the word "takeoff?"
I've been taught before that the word "takeoff" should only be used when reading back a clearance, and that when requesting a clearance pilots should advise "ready" or "ready for departure."
Here's the really critical point to the question. I've always believed that the word "takeoff" should only be used when reading back a clearance for such an action. Not when requesting a clearance for that action.
I've also heard it taught that this is true only for IFR flights but it's okay to ask for a clearance with the word "takeoff" when VFR. I don't see why the difference between VFR or IFR flight would be a factor here.
Has the ICAO, FAA or any other controlling organization released any written guidance on radio communications that covers this procedure?

I'm 3 years late, but here are my 2 cents. I understand controllers not using the term "takeoff" except when they are clearing an aircraft for takeoff. However, in the 90ies, in Southern California I was taught, as a pilot, to use the term "take-off" in two situations. 1) requesting "taxi for take-off"; and 2) at the runway hold-short line when i'm "ready for take-off".

@Devil07, my two cents: Don't do that.

The word "Takeoff" should only be used when clearing somebody for takeoff, acknowledging your takeoff clearance, or cancelling/acknowledging a cancelled takeoff clearance.
"Departure" should be used in all other circumstances, and as far as I'm aware whoever told you you could use the word Takeoff for VFR was wrong.
Anybody saying the word "takeoff" intends for an aircraft to enter, accelerate along, and then leave a runway in the immediate future: never anything else. Anybody saying otherwise should be immediately directed to the Wikipedia article for the Tenerife 747 disaster.

Do you have any references?

Yeah @RyanBurnette I'm struggling to find a regulation, and indeed I'm not sure if it's been codified - if it has I can't find it.

I get the relevance of the Tenerife disaster but it's not the type of source I'm searching for on the topic.

I think the point is that you'll find the answers around that disaster - the recommendations came in the investigation and accident reports etc

I've also heard "wheels-up" used to be more specific than "departure."

Typically sitting at the hold-short line I call tower and say "Tower, 12345 at XX (Intersection) Ready To Depart"

@RyanBurnette The examples you'll find in the FAA's Aeronautical Information Manual all use the "ready for departure" phraseology - there used to be several but seems like just one now. I'm not aware of anything regulatory or in the Pilot/Controller Glossary that says we shouldn't use the phrase "ready for takeoff", but it seems general practice is to avoid using that word unless reading back a clearance (using "ready for departure" or "ready to go", or any number of other circumlocutions to avoid the "T" word).

@voretaq7 I have noticed that all the AIM's example use the "ready for departure" phraseology and any examples of "ready for takeoff" are contextual and not examples of radio communications. I'm leaning towards deciding this is one of those things that doesn't have a correct answer, but I'll prefer and teach the procedure that makes the most sense to me.

It has a correct answer, it's just part of the procedures rather than the regulations

@JonStory Fair point.

YES YES YES, I can't stand being in final approach and hear the cool kid in the block say "hey Jay let me go a minute at the bar and then we'll takeoff", scares me every time.

ICAO Doc. 4444 specifies standards for ATC communication. "DEPARTURE" is used for taxi-ing etc, and "TAKE-OFF" is used when actually taking off.
In Europe, this EASA guide specifies much the same thing.

I'll check out ICAO Doc. 4444, but can you elaborate on what it says? My question is specifically about how to call and ask for the clearance when you're ready at the hold short line or in sequence.

There's no definitive FAA statement one way or another that I could find. Outside the US and per ICAO, "take-off" is only used when accepting a take-off clearance and you should use "departure" in other situations.
There's nothing I could find about departure vs. take-off phraseology in the P/CG or AIM. The ATC orders 4-3-1 do mention it, in the context of clearances:

4−3−1. DEPARTURE TERMINOLOGY
Avoid using the term “takeoff” except to actually clear an aircraft
  for takeoff or to cancel a takeoff clearance. Use such terms as
  “depart,” “departure,” or “fly” in clearances when necessary.

The AIM uses "departure" in its examples but makes no comment about avoiding "take-off" and an FAA runway safety publication doesn't have anything to say about it either. AOPA has an article (from 1999, admittedly, so perhaps it's now considered outdated) that says that pilots can use "take-off" to tell ATC they're ready:

When you're ready for takeoff, use the same "who, where, and what"
  format for your initial call to the tower - "Anytown tower, Trainer
  zero-zero-zero-zero-Yankee , ready for takeoff Runway three-three."

That phraseology is commonly used at my local airport in the US but it would be a major no-no in South Africa, where I learned to fly. And Jon's answer is based on UK experience (only because it's in his profile), so it looks like an ICAO vs. US thing, at least to some extent.
And indeed, ICAO document 4444  uses the word "departure" as part of "preparation for take-off", and "take-off" itself for "take-off clearances" (see sections 12.2.4.10 and 12.3.4.11). 12.2.4.10 says that pilots only need to say "ready" to inform ATC that they're ready for departure: "Tower, N12345 is holding at 18, ready". It doesn't address uncontrolled fields, but the usual call I was taught was "N12345 is departing runway 18".

"Take-off" indicates to others on frequency that a potentially high-risk operation is about to be undertaken by another aircraft, which they should take a special interest in for situational awareness and conflict management.
Procedures vary by field, but standard ICAO phraseology (or equivalent for the jurisdiction) should be employed for all the radio telephony. Assuming the aircraft is waiting on a taxiway at the active runway, a typical notification to the tower that the crew are ready to depart might be:

Some Tower. Speedbird Niner Juliet. Holding runway 27 left, ready for departure.

Typical responses from the tower would be:

Cleared for takeoff [runway], wind [...]. The aircraft is cleared -- at the crew's discretion -- to taxi onto the designated runway, apply power and lift off.
Runway [identifier], line up and wait. Cleared to taxi onto the designated runway, DO NOT commence take-off roll, await further instructions. (Delays typically for spacing / sequencing, conflicting traffic movements, departures taking place behind arriving aircraft, ...)
No immediate action. The aircraft cannot enter the active runway for some reason, so continues to wait at the hold-short line until further instructions or clearances are provided.

At some fields, it may not be necessary or desired to provide an indication of departure readiness. Ground may provide such indication on the pilot's behalf and advise at switchover to simply monitor the tower frequency.
Non-standard RT phraseology does often creep in. I sometimes hear either "[Callsign] rolling", "rolling [runway identifier]" or a combination announced upon commencing the roll. This is particularly the case if there has been a delay between issuing the take-off clearance and the departure commencing. It's a courtesy and notification to others on the field or in the pattern to watch out for the metal about to be thrown along the tarmac at high-speed.

The ICAO Common Taxonomy Team defines take-off as:

From the application of takeoff power, through rotation and to an altitude of 35 feet above runway elevation. This phase of flight includes the following subphases:
• Takeoff.  From the application of takeoff power, through rotation and to an altitude of 35 feet above runway elevation or until gear-up selection, whichever comes first.
• Rejected Takeoff.  During takeoff, from the point where the decision to abort has been taken until the aircraft begins to taxi from the runway.

So, the term 'takeoff' should be used only if the intention is to acclerate, rotate and leave the runway. Use of takeoff in any other sense would result in confusion, with potentially dangerous consequences.

That document applies to accident and incident reporting terminology, not radio communication phraseology.


text:
What type of Flare is best used in GA Survival kit?
I have a question regarding Survival Kit Signaling equipment for a General Aviation aircraft. what type of emergency Flares are recommended for aviation use? The Signaling gun with cartridges type or another type? I would appreciate if anyone could point me in the direction of some good Emergency flares for aviation Survival Kits.

It depends on your operations (day/night/land/water) but this AC has some really good info on various types to cary in different situations. Although its important to note that flares are dangerous and flammable making them potentially hazardous in an aviation situation:

The Federal Aviation Administration (FAA)  recognizes that many pilots
and operators of aircraft are  utilizing some distress signaling
devices that are inherently  dangerous, as well as ineffective, when
used in actual  emergencies. Typical of the problem is the use of
"railroad or  highway flares," which produce an excessive amount of
high  temperature slag, usually burn the user when hand held, and are
easily extinguished when used in open bodies of water. Further,  the
incendiary nature of these and other pyrotechnics presents  problems
for their safe and proper storage in the aircraft  environment. This
advisory circular is written to guide the  pilot/operator in the
proper use and identification of  pyrotechnic signaling devices.

This survival kit includes both hand held and launching flares and seems to be marketed for aviation.


text:
What should a pilot do in this lost communication scenario?
As far as I study, when a pilot faces a lost communication problem, the pilot should consider MEA (Minimum Enroute Altitude, Expected Altitude, Assigned altitude) for altitude and AVEF (Assigned heading, Vector, Expected route, Filed route) for route. I am confuse about what V (Vector) means.

For example, I have filed:

DVT 1 departure->PHX VOR-> V105 ->DRAKE(PRC VOR)->V257->BISOP->GRAND
  CANYON Airport

I got a clearance like this:

555AZ cleared to Grand Canyon Airport via DVT one departure PHX vor
  vector to Drake Altitude 10000' Frequency 120.7 Transponder 3435.

However, my communications fail before I arrive at PHX VOR. In this situation, A (assigned) is [DVT one departure->PHX VOR-> DRAKE]. What about V (vector)? Is it DRAKE? Or if not, how can I manage this situation? I think I should go to PHX VOR via DVT one departure. When I arrive PHX VOR, I go to DRAKE (PRC VOR) directly then go to fly as I filed. Is it correct? If not, could you let me know?

Vector means a vector that you are on—not one you expect. In your case, you have no idea when (or if) you will be vectored to Drake so there is no vector to fly. So fly the filed.

If you can maintain VMC, then I believe you'll proceed VFR to the nearest practical landing (perhaps SDL, or return to DVT).

It means to expect radar vectors between PHX and DRAKE because ATC expects to have to move you around to accommodate the traffic situation.  If you had a comm failure, once you squawk 7600, ATC will expect you to find your way between PHX and DRAKE by the most logical path.
In this case, because you had filed to fly along V105 you would take that route and that's what the controller would expect.  The controller would quickly figure out what you are doing as you proceed down V105 and would be able to route other traffic to maintain separation to accommodate your track.
The only thing that would create a problem is if you headed off in some direction that made no sense to the controller once at PHX.


text:
Need help Identifying Aircraft Panels
I have the following aircraft panels which I would like to Identify and get an understanding of its value.
Any help is greatly appreciated!
First Panel:
Length: 19 ½ in.
Height: 8 in.

Second Panel:
Dimensions:
Length: 15 ¼ in.
Height: 10 in.
Panel Dated March 16, 1965
Metal panel with a matte grey finish.
Top right of panel reads “Marker Beacon” “FM/Z, OM, MM”

Third Panel:
Dimensions:
Length: 18  in.
Width: 11 ½ in.

Fourth Panel:

You might want to make your dimensions more consistent. Some are "Length/Height", some are "Length/Width" and one has no dimensions at all. I'd suggest "Height/Width" for all of them, as "length" just doesn't really seem to apply...


text:
Why do pilots announce the cruising altitude of a flight?
At some early stage in the flight, why will commercial pilots announce the cruising altitude of the flight?

So if you fall out, you know how long before you hit the ground! :)

Obligatory comic (http://theoatmeal.com/comics/airplane_heard): "What am I supposed to do with this information? Oh gee that's my favorite altitude!"

No real reason other than it keeps people from bugging the flight attendants by asking "how high are we??".
It also lets the flight attendants know that the aircraft is at cruise so won't be climbing and making the carts harder for them to push around.

Passengers like information and even if some do not, it isn't much an inconvenience to hear it anyway.  That information is a matter of tradition as far as I can tell.  It is written nowhere in my manuals what is to be said in the boarding PA except some general guidelines on how to conduct yourself on the PA system.  PA announcements you've heard have the cruising altitude, and the captain showing a new first officer how to do the boarding announcement will include that information.  That first officer will go on to include the cruise altitude and when he is a captain he'll pass it along to his first officers.
If you took a look at every pilot individually you'll find that the vast majority of each pilots boarding announcements are exactly the same (aside from things that vary like the specific cruise altitude and weather information).  After saying the announcement a few times you settle into a routine and it perpetuates.

If it is at the beginning of the flight, I would guess they are just sharing in case any passengers are interested.
It may give flight attendants an idea of how long it will take to reach cruise.


text:
Why would an airport use crossing runways for takeoff and landings?
I was listening to some RealATC this evening and came to this one about an accident at LaGuardia airport. This question is not about the accident in any way, but I am using this video because it has an example of what I mean and has a labelled diagram as well.
As the clip advances, it becomes clear that at that time, planes were landing via ILS Runway 13. However, planes were being directed to takeoff from Runway 4.
Under what circumstances might a major commercial airport have planes landing and departing via crossing runways, instead of on the same, or at least parallel?

They need the throughput.
Got to use what you have. They utilize their crossing runways to the full under any conditions that allow it.
Major commercial airports try to have as many parallel runways as possible, as crossing runways are a hassle. (See DFW, ATL, LAX, many other examples of new or remodelled major airports using almost exclusively parallel runways). But sometimes (for any number of reasons, a main one at LGA being space) parallel runways haven't been built, so they can't be used.

Any time the winds and traffic permit an airport may chose to use intersecting runways. You can read some interesting info on it here and here. In some cases when both runways are in use they may have the aircraft execute Land And Hold Short procedures. The pilot has the final say in a LAHSO situation and you can call no joy if you think you cant complete the maneuver as requested.
Some airports may only have 2 runways set up in an intersecting layout and may not have the luxury of parallel runways. Depending on terminal layout at commercial airports using both runways may aid in traffic flow on the ground as well as getting planes in the air.
Winds permitting generally have to do with the maximum cross wind component of a given airplane. Although generally speaking you will see this done in calm wind situations. The other cause I have seen for this is instrumentation, some airports may have intersecting runways that are not equipped the same or of the same length. As lets say the favorable runway is is 5000ft visual approach only strip and you need 6000ft to land. Luckily the airport has another intersecting runway that is a 7000ft strip (and has an ILS approach not that you need it (its nice out)). They may have you come into the other runway while they sequence small visual traffic on to the short strip (possibly having them land short if need be).

Another reason for using crossing runaways is sound reduction. For a time, Barcelona Airport had a runaway parallel to the coast and another leading to the sea. When possible, the former was used for landings and the latter for take offs. This way, the loudest sounds were produced over water.


text:
Why did this flight descend way below usual path?
I flew on UAL2086 last night which is a regularly scheduled flight from KIAH to KLAS, this particular plane was a 737-900 - N79402. We took off early and had no issues until we arrived in Arizona when we descended to 20000 ft the flight attendants asked everyone to be seated for safety reasons. There was some bumps which could’ve been turbulence but overall didn’t seem serious. Capt eventually came over intercom asking the FAs to check in. Nothing else was really said until we were 70 miles or so from KLAS. Upon arrival the FAs said there would be airport staff waiting with any questions we may have had regarding the incident but when leaving the plane no information was given or known. We stayed at 20000 ft well before usual descent procedures as we were still over 300 miles out. I reached out to United regarding what happened but does any of this seem odd to you guys?

@MichaelHall I believe that is what the OP is asking...what happened. If they were not told of what happened by UA, I would say that the best choice is to see if Audio is available for the flight on one sites that do that. However the second screen shot is confusing because it doesn't cover the period of time which whatever occured. The OP should update the second screen shot to show what happened between 930pm and 10:00pm. not during the initial take off.

I apologize as it won’t allow me to add anymore photos beyond one. I’ve since edited it and added when the descent began. I’ve looked for audio but haven’t found anything yet.

ahh yeah limitation of a new poster. the top chart with speed and altitude is more important but the other stuff shows about a 750 fpm descent from 30k to 20k.  just an additional point on the upper chart the sequence of the descent looks kinda normal, but speed isn't as smooth as I would guess it should be for a controlled descent. the speed briefly increases when the descent is initiated, then is a bit unstable until acceleration is used to stop the descent. after the descent to 22k and later 20k.  velocity which I'm thinking is ground speed smooths out, so maybe they lost an engine.

That tail number was flying today on normal routes so not sure why the fast descent. https://www.liveatc.net/search/?icao=klas on that site you can listen to archived audio though a small link in each section.  IAH to LAS would have arrived from the South East so that is where your flight is lrobably recorded.

Previous flight schedules with this route and various other routes show this was out of norm. The lowest I’ve found was a flight from the last two weeks that descended to 28k for approximately 11 minutes before entering true descent phase. Traffic wasn’t a issue unless it was something not showing on radar. I’m a frequent flyer and a aircraft mechanic and this was not normal to me.

It's very telling that the airline admitted that there was an "incident", yet didn't say what it was.  Puzzling... no unusual pressurization changes?

Quick skim through the QRH shows: 1. LE FLAPS TRANSIT, descend to FL200 and cycle flaps. 2. Elevator tab vibration, reduce airspeed until it stops, 15 degree bank angle limitation above FL200. 3. Engine restart, but the OP should have noticed that.


text:
What are all the frequencies on the sectional chart?
See the image below of a screenshot of DFW - Dallas Fortworth Sectional chart.
There are a multiple frequencies. Based on what I learnt on Sportys so far, a CTAF has a (C), then UNICOM has a UNICOM written after the frequency.
How should one know which frequency is what, when someone sees a sectional like the one in the link. Especially, where the Control Tower frequency, and/ or arrival, departure frequency etc.

https://ibb.co/s1YWhCT

Have you looked through the Chart User's Guide to see what each frequency is? If so, what exactly isn't clear? I'm not sure if you're asking how to read the chart, or how to know which frequencies you'll need during a flight?

As Pondlife said, the information you want is found in the Aeronautical Chart User's Guide — VFR Charting Products publication.
Control Tower frequencies are the ones shown in blue Airport Data Groupings with CT - in front of them.
Approach Control frequencies that are relevant to VFR pilots are shown in blue (Class B) or magenta (Class C) boxes at the outskirts of Class C and Class B airspace, but are otherwise not shown on the "map" portion of the sectional. However there is a listing at the edge of the sectional/TAC of Class B, Class C, TRSA, and Selected [i.e. certain Class D] Approach Control Frequencies. Note that if you are using an online map like SkyVector you will have to select a specific sectional instead of "World VFR" in order to see the outer non-map areas.
If there is a specific frequency on a specific chart that you have a question about after consulting the Chart User's Guide you can edit your question to describe it.


text:
What is the time-lag between an airline's decision to serve a route and the starting of its service?
I was wondering whether there is any literature about the time-lag between the moment in which an airline decides to enter a given route and pays the associated fixed costs and the moment in which the airline starts to sell tickets and fly in that route.
Could you point out some common sense narrative and also some more formal studies (if any)? How does the time lag compare to other industries (e.g., car manufacturing)?

I work for an airline. Normally it takes no less than 6 month from the GO to maiden flight. It depends on many factors, but simplified, it goes like this:
When Commercial teams look for new opportunities, they prepares a "business case" balancing the profitability of the route and costs. Main board evaluates the proposal and gives the GO.
It is very hard to open a new route before 6 months, as all airplanes are assigned to other routes with already-sold tickets. Assigning aircraft into the new route requires a very delicate coordination to maximize each aircraft's utilization and avoid unused ground times.
Having that, you can start to sell tickets. At the same time, legal, maintenance, operations and airports teams have to run to get in place all certifications, personnel, permits, contracts and requires stuff for the route.
(Prior to all this, it is important you have an available slot in the origin/destination airports, otherwise it could take much longer.)


text:
Variable versus fixed costs in the airline industry
I want to list the costs airlines have to sustain by distinguishing them between variable and fixed. Variable costs change in proportion to aircraft usage. Fixed costs show little or no change in proportion to changes in activity.
This is an incomplete list I took from the FAA website (here)
Variable costs:

Fuel and oil.
Maintenance (e.g., maintenance labor, parts airframe/engine/avionics, engine restoration, thrust reverser overhaul, propeller Overhaul).
In-ground and in-flight crew salaries (can also be thought as fixed costs).

Fixed costs:

Parking and hangar fees.
Insurance.
Aircraft modernisation.
Computerised maintenance management program.
Weather service.

Question:
(1) In which category should I put

Landing fees (ATC, runways, taxiways, parking at stand or apron for a certain time (e.g. 2 hours), use of gates / fingers / terminal for disembarking).

Lounges fees.

Scheduling coordination.

Surcharges or rebates in addition of landing fees (for noise, peak-hour operations)

Passenger charges

Administrative cost (managing/issuing tickets, etc.)

Overflight fees

(2) I'm fundamentally confused about the distinction between fixed and variable costs. For instance, landing fees increase with the number of flights and, hence, with the demand of tickets for a given route. Should they be classified as variable costs for this reason? Is this the same reason why fuel and oil, maintenance, and crew salaries are classified as variable costs?

If the cost stays the same with or without flights, it's "fixed". If another flight increases the cost, it's "variable."  If landing or overflight fees are per-landing, that's variable. If the lounge fee is $XXX per month for as much or as little as you use it, that's fixed. "Fixed" means, essentially, a fixed amount irrespective of the number of flights. "Variable" means, varies with the number of flights. If you double the number of flights next month, the costs that (roughly) double are variable; the costs that don't change would be the fixed costs.

Thank you. Given your distinction between fixed and variable costs and according to your knowledge of the airline industry, how would you classify the items listed in my first question? I understand that it may change from airport to airport, but perhaps there are some general rules.

Also, you mention that the distinction is based on the number of flights. Is it also based on the number of airplanes? For example, I may think that, for a given number of airplanes, hangar fees are a fixed costs independent of the number of flights. However, hangar fees increase with the number of airplanes because more space is needed.

Landing, overflight, & noise seem variable. The rest depend on details... is Lounge fee "per month" or "per hour used"? Likewise ticketing & scheduling... set cost for the infrastructure before the first ticket is sold, or do they scale with the number of tickets/flights? I could see ways to pay for those that are variable (paying for a service on a "by volume" basis) or fixed (staff the office in-house). All depends on the specific details of the given arrangement.

Reference hangers, I'd see that as, "as your fleet grows, the fixed cost of (renting/maintaining) hangers increases."

This seems to be more about accounting than aviation.

I’ll address your second question first.

I'm fundamentally confused about the distinction between fixed and variable costs.

Simply put, variable costs are ones that go away if you cancel a flight (or, more likely, a pair of out-and-back flights) while fixed costs are ones you still have to pay.
A challenge is that many “fixed” costs are actually step-variable costs. For instance, if you cancel enough flights, you might be able to reduce how many aircraft or gates you need. But with the black and white dichotomy given, we classify those as fixed.

Landing fees (ATC, runways, taxiways, parking at stand or apron for a certain time (e.g. 2 hours), use of gates / fingers / terminal for disembarking).
Surcharges or rebates in addition of landing fees (for noise, peak-hour operations)
Passenger charges
Overflight fees

If you don’t land, there is no landing fee, so that’s variable. Ditto for gate fees, noise fees, peak-hour fees, passenger fees and overflight fees.
Space to park an unused plane isn’t usually an issue when it’s just one or two flights canceled; that will be included in some other fixed cost.

Lounges fees.
Scheduling coordination.
Administrative cost (managing/issuing tickets, etc.)

These are fixed (or step-variable), since canceling one or two flights won’t meaningfully change them.


text:
Hub-and-spoke networks: fixed and variable costs
In the economic literature, many authors sustain that hub-and-spoke networks:

Increase the fixed costs sustained by the airlines.

Reduce the marginal costs (by allowing for a more  efficient use of aircraft and other inputs).

Could you explain this with an example?

Is this a different question to your last? https://aviation.stackexchange.com/questions/84449/variable-versus-fixed-costs-in-the-airline-industry

Thanks. It is different. Here, I want to understand (with an example, if possible) why a hub-and-spoke network business model involves higher fixed costs and lower marginal costs than a point-to-point network business model.

The fixed cost problem with the hub and spoke model is mainly due to the need for banked flights to minimize average connection times.
Ideally, every plane starts the morning at a spoke and arrives at the hub at roughly the same time. Then passengers have an hour or two to change planes, and all the planes fly back to their spokes. This cycle is repeated again in the afternoon, and then the planes spend the night at their spoke to start the cycle again the next morning.
The problem is that this requires duplicate gate space at the hub and spokes, even though planes will only use one or the other at any given time (or neither, while flying). It also requires sufficient planes to serve every spoke in every bank even if there’s not enough demand to fill them. And it requires much larger airports and terminals at the hubs to serve massive numbers of passengers and flights during the banks, which goes unused the rest of the day.
Of course, nobody operates quite this stupidly in practice, but every flight that deviates from the model means longer connection times for its passengers because if it arrives at the hub between banks, passengers have to sit in the terminal and wait for the next bank to connect, which makes your flights less desirable than a competitor’s.
The upside is that you can (in theory) size the aircraft serving each spoke to its average demand, and likely use fewer, larger aircraft since one plane can serve every passenger connecting to every destination, which reduces variable costs.

Other downside of hubs, the aircraft sit longer waiting for passengers to shuffle (at the hub) or waiting for their departure time (at a near-to-the-hub spoke city) to make the next bank of flights. Flights operating point-to-point wait for people + bags off + on, but not necessarily much else.

Thanks. Is the upside that you highlight what is called economies of density (or scope?)? Also, given the downside story, can we say that the fixed costs at a hub are increasing in the number of arms?

@TEX The fixed cost of the hub (and aircraft) scales with the number of spokes. I’m not familiar with the terms economy of density/scope.